Many structures are susceptible to a variety of types of damage or failure. One of the most common forms of damage for many types of structures involves fatigue cracks that typically originate at holes or other fastener-type locations in structural joints. For example, heavy duty mobile vehicles such as planes, helicopters and military vehicles, and structures such as bridges and buildings, may include layers of metals that are secured together by a bolt or other fastener in a thru-hole passing through the layers. Assuring the proper fastening force during assembly of the structure, and detecting this force along with the health of the joint at the fastener location is important to maintaining the operability of these structures throughout their lifetime in service. Techniques for detecting and analyzing the status of joint health, fatigue, failure or other characteristics of materials that are fastened together, have generally involved using probes or sensors at various points on exposed surfaces of the layers or at unexposed layer interfaces.
Often, the placement and operation of the sensors after a vehicle or structure has been assembled requires that parts of the structure be disassembled. Many sensors are incapable of placement at locations that facilitate the detection of certain types of damage, such as damage at underlying layers or at structural locations that are otherwise difficult to access. Intrusive and destructive placement of sensors is often not desirable or not possible. Non-destructive approaches where sensors are placed on available surfaces often cannot provide desired analysis of characteristics of internal portions of the material under study.
These and other issues remain as a challenge for detecting structural and electrical characteristics of structures such as multi-layer structures.